Voltage controlled oscillators (VCOs) generate a periodic clock signal in response to an input voltage. There are numerous applications for VCOs such as in tunable frequency generators. Additionally, VCOs are often used in phase locked loop (PLL) circuits to generate a clock signal that is dynamically phase and frequency compared to an input reference clock signal. To keep the generated clock signal aligned with the input reference clock, the phase difference is monitored and a phase correction circuit is used to change either the phase or the frequency (or both) of the generated clock signal to match the phase of the reference clock. Designers of VCOs have been faced with the problem of providing a relatively small and easy to implement phase correction circuit that will operate at low power supply voltages (e.g. 1.0–1.5 volts).
Therefore, it is desirable to provide an integrated circuit VCO having a phase correction circuit that is small and easy to implement.